Change AddList to take a completion callback.

 AddList now takes a completion callback. This callback is invoked when
 distillation is complete with the status of distillation. Clients can
 add an item to the list and then use callback to update their state
 when distillation finishes or gets cancelled.

 BUG=288015

Review URL: https://codereview.chromium.org/105393008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241768 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241768